This research program has been constructed with a primary aim of investigating the controlling events concerned with normal and abnormal cell development processes. This program will bring together studies utilizing both immunologic and non-immunologic systems for approaching questions pertaining to differentiation, activation and expression of biological function of mammalian cells. The diversity of scientific disciplines among the staff scientists involved will be integrated in the context of the theme of the program. Moreover, although each individual research program constitutes an entity within itself, there are ample, as well as necessary, collaborative interactions between related programs. Studies to be undertaken include the role of certain soluble mediators in controlling various aspects of cell differentiation and function and physicochemical analysis of such mediators (Research Program I), analysis of the diversity of sarcoma gene products and the relationship of this diversity to normal cell development and malignant transformation (Research Program II), delineation of the mechanisms of diversity of immunologic receptors on both classes of lymphocytes (Research Programs III and IV), analysis of host-tumor interactions and development of new methods for favoring host rejection of tumor growth (Research Program V) and structural analysis of murine retroviral gene products and the relationship of recombination events to leukemogenesis (Research Program VI). Core facilities to serve all of the research programs include a hybridoma facility and fluorescence-activated cell sorter facility. The collective information obtained in this program should broaden our fundamental understanding of differentiation mechanisms and of how disturbances in such mechanisms may result in abnormal cell development and function.